1. Field of the Invention
This invention relates to hip stem prothesis apparatus and, more particularly, to a novel bone milling guide apparatus and method for the precision milling of a socket into the medullary cavity of the proximal femur of a patient, the socket thereby being prepared to receive a hip stem in a close-fitting relationship.
2. The Prior Art
Total hip replacement is one of the most remarkable advances in orthopedic surgery of this century. Since the first total hip joint replacement in 1962, significant advances have been made in both implant design and surgical technique. These improved devices and procedures offer new hope for patients crippled by degenerative arthritis, rheumatoid arthritis, or significant trauma to the hip. Diseases such as rheumatoid or osteo-arthritis generally result in degradation of the cartilage lining the acetabulum so that the ball of the femur rubs against the ilium. This rubbing action causes pain and further degradation of the remaining cartilage. Bone erosion causes the affected bones to attempt to compensate by reshaping, thus resulting in a misshapen joint which may eventually cease to function altogether.
Total joint replacement can provide not only marked resolution of pain but significant functional improvement. Currently, approximately 250,000 successful total joint replacements are performed each year in the United States alone so that the replacement of a hip joint with an artificial implant or prosthetic device is now a routinely practiced surgical procedure. The long-term success rate following total hip replacement is excellent. It is estimated that over 90% of patients who have had total joint replacement are functioning well 12 years after surgery.
A conventional hip prosthesis consists of an artificial femur head or ball mounted on the neck end of a stem with the ball being received in a prosthetic acetabular socket affixed to the ilium. The proximal end of the femur is removed and the stem is anchored in the medullary bone cavity. Despite overall excellent results, problems may infrequently develop following total joint replacement. The major potential complication of total joint replacement is infection. Pain following total joint replacement may also be due to mechanical loosening or breakage of the implant resulting in excessive motion between the prosthesis and the underlying bone. In relatively rare instances, a second, total joint replacement or revision may be required. It is estimated that approximately five percent of all total joint replacements performed today are revisions of previous procedures.
One of the major problems encountered during joint-replacement surgery is the need to securely anchor the hip stem portion of the prosthesis in the medullary bone cavity. Numerous attempts have been made to solve this particularly vexing problem. Early procedures involved reaming most of the cancellous bone from the proximal end of the medullary canal followed by packing the resulting cavity surrounding the prosthetic hip stem with bone cement so as to assure fixation between the hip stem and the surrounding cortical bone. Bone cement was necessitated because the metaphyseal geometry does not necessarily have any relationship to diaphyseal geometry, and it was found to be virtually impossible to predetermine the precise configuration for the prosthetic device. Accordingly, the customary practice was to use the cement material to achieve fixation by using it as a filler between the hip stem and the adjacent cortical bone. Unfortunately, revision is also rendered considerably more difficult by the presence of bone cement.
Clinical experience by a noted orthopedic surgeon over the last decade has demonstrated that, in the short term, cemented arthroplasties are more forgiving than those designed for biologic fixation. For example, it was found that it was rare for a patient with a cemented arthroplasty to experience clinical symptoms of fixation failure within the first few postoperative years. In contrast, a technically poor insertion of a porous-coated implant often results in fixation failure from the very start, with patient dissatisfaction as soon as weight bearing is allowed.
A more suitable alternative that has evolved is that of a noncemented total hip replacement wherein the prosthesis is implanted in the absence of a cement. The potential for adequate bone ingrowth to create an enduring cementless implant fixation can be realized only if stable fixation is achieved from the start, particularly since fixation through bone ingrowth succeeds or fails within the first several months after implantation. The most important prerequisite for secure fixation and better physiological stress transfer between implant and osseous tissue is initial mechanical stability. Micromotion between the implant and the surrounding osseous tissue into which it is inserted must be minimal during the time when the intramedullary fracture callus adjacent the implant is differentiating into osseous tissue and maturing. This initial mechanical stability can only be achieved with careful preoperative planning, meticulous surgical technique and a wide selection of incrementally sized hip stem components.
The fundamental problem is still that of the range of anatomical variations encountered in the femur. Basically, the medullary cavity of the femur is in the shape of an inverted, triangular pyramid at the top and a rod at the bottom. The first problem during preparation of the medullary cavity is to match the diaphysis which can be done by simple reaming to define the size of the stem. The second problem is to match the proximal end of the stem to the cortical bone. One approach is to provide the stem with a size range of sleeves which can be mounted to the stem in a locking relationship using a conventional Morse taper. Not only does the Morse taper allow one to use a preselected sleeve size, but it also accommodates placement of the triangular portion of the sleeve at a preselected angle to the neck of the stem.
From the foregoing it can be readily seen that the preparation of the proximal end of the femur to receive the proximal end of the hip stem is the major challenge. One bone milling device is disclosed by Frey et al (U.S. Pat. No. 4,777,942) and includes a milling instrument having a caliper that is inserted into the medullary cavity. A spindle is linked to the caliper at an angle and carries a milling cutter as well as a guide shoe at its distal end. The guide shoe slides within a guideway on the distal end of the caliper. The instrument guides the milling cutter to cut a circular arc corresponding to the boundary line between the spongiosa and cortical tissue in the region of the calcar arc.
Forte (U.S. Pat. No. 4,306,550) discloses a combination of tools and methods used to prepare a socket in a femur for receiving a femoral prosthesis. A rasp is used to form a socket in the femur. A cutter is journaled to the rasp prior to its removal and is rotated to machine the surface of the calcar surrounding the socket.
Experience has shown that these prior art devices are complex and require extensive experience before they can be used with any suitable degree of accuracy. Further, even experienced surgeons must rely heavily on personal expertise to accommodate for the fact that neither of these prior art devices accurately control the preparation of the medullary cavity or socket with a suitable degree of precision.
In view of the foregoing it would be an advancement in the art to provide a bone milling guide to enable the surgeon to easily and accurately mill the proximal end of a femur to receive a hip stem prosthesis. It would also be an advancement in the art to provide a bone milling guide that is simple to use and is mountable in the socket prepared to receive the distal stem of the prosthesis. Such a novel apparatus and method is disclosed and claimed herein.